(2,2)-paracyclophane is a well-known product and it is mainly utilized in the preparation of poly-p.xylylene, a high-performance product because of its capability of coating surfaces.
Various processes have been suggested for preparing (2,2)-paracyclophane (I). Such known processes, however, are not fully satisfactory and are not suitable for industrial use, owing to the low yield of the process and to the difficult recovery of the product from the reaction mixture.
Thus, for example, "Organic Syntheses", Coll. Vol. 5, John Wiley & Sons, Inc., New York/London/Sydney/Toronto, 1973, pages 883-886, describes a process for preparing (2,2)-paracyclophane by Hofmann elimination from p.methylbenzyltrimethylammonium hydroxide obtained by reaction of the corresponding bromide with silver oxide. The elimination was carried out in the presence of an inert organic solvent (toluene) and a yield of 10-11% was obtained, with formation of undesired polymeric by-products difficult to separate.
According to European patent application No. 108,297, corresponding to U.S. Pat. No. 4,532,369, the reaction yield can be raised if Hofmann elimination occurs in the presence of high amounts of dimethylsulfoxide. This process permits to obtain high yields (about 70%), but the dimethylsulfoxide recovery makes the process relatively little attractive for industrial applications. Furthermore, the (2,2)-paracyclophane prepared by this process is not fully satisfactory as regards its purity degree.
Generally, in all the known processes for preparing (2,2)-paracyclophane, considerable amounts of poly-p.xylylene are formed, which, in the presence of the organic solvent in the reaction medium, assumes a gelatine-like appearance and is not readily separable by filtration. To overcome this drawback it would be advisable to conduct the Hofmann elimination reaction in the absence of any organic solvents. However, from British Pat. No. 807,196 it is known that p.methylbenzyltrimethylammonium hydroxide, when treated at temperatures higher than 100.degree. C. in an aqueous solution of an alkaline hydroxide, essentially generates poly-p.xylylene.